Error "Resource temporarily unavailable" occurs during slot migration by raw-key-value type.

### Search before asking

- [x] I had searched in the [issues](https://github.com/apache/kvrocks/issues) and found no similar issues.

### Version

since 2.8

### Minimal reproduce step

1. set migrate-type as raw-key-value
2. set migrate-batch-size-kb larger than 10240
3. set migrate-batch-rate-limit-mb larger than 64
4. migrate a lot of data in a slot-range.

### What did you expect to see?

migrate success

### What did you see instead?

migrate failed, error "Resource temporarily unavailable" occurs.

### Anything Else?

_No response_

### Are you willing to submit a PR?

- [ ] I'm willing to submit a PR!

Start by reproducing slot-range migration with raw-key-value, a batch size above 10240 KB, and a rate limit above 64 MB. Trace the migration path where the "Resource temporarily unavailable" error is returned; done means the migration completes successfully for the reported configuration.

Written by the indexing model from the issue text.